There are many kinds of railroad cars for carrying a lading of particulate material, be it sand or gravel aggregate, plastic pellets, grains, ores, potash, coal, or other granular materials. Many of those cars have an upper opening, or accessway of some kind, by which the particulate is loaded, and a lower opening, or accessway, or gate, or door, by which the particulate material exits the car under the influence of gravity. While the inlet opening need not necessarily have a movable gate, the outlet opening requires a governor of some kind that is movable between a closed position for retaining the lading while the lading is being transported, and an open position for releasing the lading at the destination. The terminology “flow through” or “flow through railroad car” or “center flow” car, or the like, may sometimes be used for cars of this nature where lading is introduced at the top, and flows out at the bottom.
Discharge doors for coal gondola cars or other bottom dumping cars may tend to have certain desirable properties. First, to the extent possible it is usually desirable for the door opening to be large so that unloading may tend to be relatively fast, and for the sides of any unloading chute (e.g. slope sheets) to be relatively steep so that the particulate will tend not to hang up on the slope. Further, to the extent that the door can be large and the slope sheets steep, the interior of the car may tend to have a greater lading volume for a given car length. Further still, any increase in lading achieved will tend to be at a relatively low height relative to Top of Rail (TOR) and so may tend to aid in maintaining a low center of gravity. A low center of gravity tends to yield a better riding car that is less prone to derailment, and perhaps less prone to cause as much wear or damage to tracks. Some cars, such as ballast cars, or cars designed for releasing lading between the rails, may tend to benefit from having discharge doors that are oriented longitudinally, such that the discharge lip of the door runs substantially parallel to the longitudinal centerline of the car, and, in opening, the motion of the door may tend to be predominantly in a direction transverse to the centerline of the car.